The present invention relates to single-chip semiconductor light emitting diodes (LED), and more particularly to single-chip LEDs as high voltage alternating current (AC) or direct current (DC) semi conductive light emitters coupled together to form a solid-state LED lighting lamp (SSL-LED lamp) with improved illumination capabilities.
The popularity of LEDs in various applications where illumination is needed is on the rise. Although traditionally utilized for indicator lights, background display illumination and other applications where low-level illumination is sufficient, the realization of high brightness blue/green and violet LEDs made from the III-nitride semiconductor family InN, GaN, AlN and their alloys in recent years may enable LEDs to provide sufficient illumination for general lighting applications in residential houses and commercial buildings. One successful method of producing light emissions from LEDs that appear to the human eye to be “white light” is to coat blue LED chips made of III-nitrides with yellow phosphors, such as cerium doped yttrium aluminum garnet (YAG:Ce) crystals in viscous adhesive. The LED chip emits blue light, part of which is converted to visible spectrum yellow light by the YAG:Ce. Through color mixing, and the stimulation of the red and green receptors of the eye by the yellow light, the eye sees the while light when the yellow and blue colors are properly balanced. Because solid-state LEDs are quite efficient in light output per watt of power consumed, and are sufficiently versatile to be capable of use in a variety of locations where illumination is needed, LED lamps have the potential to replace traditional incandescent or fluorescent lamps in many lighting applications. The use of LED lamps, therefore, will enable overall electrical energy consumption to be reduced, thereby lowering the output of carbon-related pollution into the atmosphere. Furthermore, materials used to form LED chips and associated coatings allow LED lamps to be independently tunable in terms of light intensity and visible spectrum color output.
Despite the potential for widespread usage of LED lamps as significant illumination sources, and improvements to the light intensity levels provided, many obstacles still exist to their implementation in many general lighting applications. Conventional semiconductor LEDs operate under low DC voltage (about 1 to 5 volts) with limited current flow (around 20 milliamps), thereby providing a fairly low luminance level. To achieve a high luminance for general lighting applications, semiconductor LEDs would need to run on a much higher input power. Thus, there are efforts to create so-called “high powered LEDs” that run on higher input power generated by high DC or AC voltage. One implementation of a high AC voltage LED lamp is disclosed in U.S. patent application Ser. No. 10/279,296, filed on Oct. 24, 2002, and entitled “Light Emitting Diodes For High AC Voltage Operation and General Lighting”, the teachings of which are incorporated herein by reference. The '296 application teaches combining many small LEDs onto a single chip with serial interconnection in achieve a single light emitting device, or lamp. The LED lamp can be configured to have a running voltage of 12V, 24V, 110/120V or 220/240V, or still other values. AC voltages of 110/120 or 220/240, for instance, make the LED lamp compatible with running on standard power sources supplied to houses and buildings in North America or in Europe and Asia, without the need for power converters. This is achieved in the invention of the '296 application by having two columns of individual LEDs each in serial interconnection and wired in opposing polarities such that one column of LEDs is forward biased and the other column is reverse biased, as seen in FIG. 1. The AC current turns on and off the two columns alternately through a 60 Hz cycle so that all the LEDs of the single-chip lamp appear to the naked eye to be illuminated all of the time. Because a sufficient number of LEDs are connected in series, in the case of an AC power supply of 120V about 35 emitters in each series column, each individual LED will have about the same voltage crossing (e.g., about 3.4V) as with a conventional DC operated LEDs, avoiding the danger of greatly exceeding the turn on voltage of the LEDs and causing damage thereto. Another advantage of this design is that power converters, transformers, large PCB boards, and other bulky electronics are not needed to operate with standard 110V or 220V AC current. Therefore, single chip LED lamps can be integrated with standard Edison or European screw bases for direct placement in conventional light bulb fixtures.
The evolution of LED lamp design to incorporate high voltage AC power sources, however, has created another set of issues to address. Semiconductor LEDs still only have a power conversion efficiency of around 10% to 35%, and thus most of the input power is dissipated as heat energy. The p-n junction of the diode, is susceptible to excessive temperatures, which cause the power conversion efficiency to drop and shorten the life of the LED emitter. Because of the difficulty of thermal management, single-chip LEDs have been limited in power consumption (e.g., to about 2 W), and thus emitted light intensity.
Still further, larger semiconductor die sizes utilized with LEDs running under a high current can have a negative effect on light extraction efficiency. Once the input voltage has increased over the value of the turn-on voltage, the current flowing through the LED increases exponentially with the increase in voltage. As input power increases, current will increase at a much faster rate than voltage. The current density level flowing through p-n junction of the LED, however, is limited based on the quality of materials used in the LED. Excessive current density can cause hot spots and non-emissive recombination sits, degrading light output and the lifespan of the device. Therefore, LEDs operating under high current (or “high powered” LEDs) must spread the current over larger p-n junctions, which typically require die sizes at least in the 1×1 mm2 to 2×2 mm2 range. Increasing the die size beyond the aforementioned range would theoretically aid in lowering the p-n junction current density with high powered LEDs, but would also have the effect of reducing the light extraction efficiency from the sidewalls of the die, thereby lowering the overall luminance of the LED. The thermal dissipation of an LED with a large die size and high power is another challenge.
High powered AC LED lamps utilizing serially connected columns of individual LEDs are also at risk of failure due to open circuits or short circuits formed by the individual LED in the series. If one individual LED forms an open circuit, all other LEDs in this series will not work either; if one LED forms a short circuit, the remaining LEDs in the path will encounter an increased voltage and risk of being damaged.
In the following description, “individual LED or “LED” means a single p-n junction light emitting diode; a certain number of individual LEDs are integrated on a single chip to form a high voltage AC/DC “emitter”; a certain number of AC/DC emitters are assembled together with a suitable package and thermal dissipation characteristics in accordance with certain embodiments of the present invention to form a “SSL-LED” lamp, as will be more fully explained herein.